An (Al,Ga)As/GaAs heterostructure bipolar transistor with nonalloyed graded-gap ohmic contacts to the base and emitter

Graded regions of n-(Ga,In)As and p-Ga(As,Sb) were incorporated side-by-side as emitter and base contacts, respectively, into an n-p-n (Al,Ga)As/GaAs heterostructure bipolar transistor (HBT). The process involved two separate molecular beam epitaxy (MBE) growths, leading to base contact regions that were self-aligned to the emitter mesas. The devices could be easily probed with pressure contacts even prior to any metallization, and excellent characteristics were obtained after final metallization. Contact resistivities of 5 × 10^-7and 3 × 10^-6Ω.cm²were measured for n- and p-type graded-gap ohmic contact structures, respectively.